Intentional and unintentional openings in a building's envelope provide opportunities for unwanted pests to enter buildings. The brown marmorated stink bug, Halyomorpha halys (Stål), is one such pest, causing a significant domestic winter nuisance in many locations. One important means of pest control is exclusion, or blocking openings through which they can enter, although some openings are intentional and cannot be completely blocked without putting a building at risk. To help understand what size openings are relevant to entry, adult H. halys ready for overwintering were driven out of heated boxes through openings designed to limit passage by their lateral pronotal and dorsoventral dimensions. Pronotally limited holes of 8 mm wide were passed by only one female (3.3% of those tested), and no females and only one male (3.3%) passed through 7-mm-wide holes. For dorsoventrally limited slits, few (13%) of females passed through 4-mm-high slits, and no individuals passed through 3-mm-high slits. Dorsoventral heights and pronotal widths of 930 individuals collected in Virginia were measured. Females were consistently larger, with pronota averaging 8.33 mm wide to the males' 7.47 mm and heights at the point of leg movement restriction averaging 4.03 mm to the males' 3.50 mm. Based on experimental data and size data, we conclude that most H. halys individuals will be excluded by slits smaller than 3 mm and holes smaller than 7 mm.